1. Filters>Render>Clouds>Solid Noise. The size settings are a matter of preference. But make sure you check the "turbulent" box. It's also best to do this on a separate image from where you want to use the dome since this noise layer should be about three times the size of the dome you want.
   
2. Colors>Invert. This turns the black streaks into bright white streaks. Then use Colors>Levels to thin the streaks down and open up the black gaps between the streaks. How far to tweak it is a matter of preference and experimentation.
   
3. Filters>Map>Map Object. I have no idea what most of the settings do here. I just know to select "Sphere" from the drop-down menu and check the "transparent background" box. On the "Orientation" tab you can rotate the sphere on each axis to find the position that looks best. Again, this is where you get to play.
   
   
4. Colors>Color to Alpha. Set the color to black. This takes all the black out of the grayscale sphere and leaves you with just white streaks and fuzz weaving around patches of transparency. I added a strange color to the background so you could see the effect. I do not understand why my sphere looks more elliptical in this picture.
   

Creating the basic dome is the simple part. From there, it's up to you to figure out how to fit it into your work. And you might still want to add gradients to define the edges better. Check out a tutorial or two on how to create a glass sphere. Then just add this effect on top of that.

There's also no reason you have to do a sphere. It's just simple because it's there automatically. You could also use "lens distortion" or other warping tricks to bend the field into other shapes. You could cut the noise layer into sections and paste those segments onto the faces of various polygons to create a faceted shield.

Or, you can learn to control lightning and bend it into less random forms. Then you can make a handy cage for your pet dragon.

Thanks for this. I'll have to give it a try, but it looks lovely. I needed cloud cover for a planet for the cover of The Captain's Chair (http://margaretmcgaffeyfisk.com/the-captains-chair/) and it was a real pain. This would have been perfect . I'm sure another book will come up needing a cloud layer.

For a regular cloud layer I'd probably go with ordinary noise, fog, or even desaturated plasma instead of the inverted noise here. Depending on the image size the bright streaks in this can be very prominent.

Thanks Sasquatch , I followed your instructions and got some interesting results which will come in useful for future projects.


I got the same, then realised to get a sphere the original image needs to be square.

So what "map to sphere" does then is map the original shape to an ellipse with the same aspect ratio as the original image. That's very handy to know. It means you aren't stuck with spheres. You can create different curves to mimic regular hand-held shields or ship deflector shields. That makes this much more useful to me. Thanks for solving the mystery.

I didn't cover it in the tutorial, but if you use Colors>Map>Gradient Map you can get very nice colors in your dome. The bird cage picture above was colored that way with the "Deep Sea" gradient. I do the gradient map before I do the color-to-alpha to get rid of the black. That way there are still shades of gray for the gradient to map to.